FSR® Integration Guide & Evaluation Parts Catalog With Suggested Electrical Interfaces, which is enclosed herewith by reference, provides an overview of the FSR® technology along with some basic electrical interfaces using such FSRs. In particular, FIG. 17 of this document (i.e.—the FSR® document) shows an FSR® current-to-voltage converter described by Eq. 1.VOUT=VREF/2×[1+RG/RFSR]  Eq. 1
Another example is given in FIG. 18 of this document showing a simple force to frequency converter with an FSR® device as the feedback element around a Schmitt trigger. At zero force, the FSR® is an open circuit. Depending on the last stage of the trigger, the output remains constant, either high or low. When the FSR® is pressed, the oscillator starts, its frequency increasing with increasing force.
It is known from document WO 2009/070503, use of a force sensing resistor where an FSR® output which is a function of the resistance is measured. Whether a change in magnitude of the FSR® output during a time interval is greater than a threshold is determined. A touch applied on the FSR® is detected during the time interval if the change is greater than the threshold. This document nevertheless presents some drawbacks among with the fact that the FSR® is a pre-loaded sensor that does not take into account the environment in which the FSR® is integrated decreasing the reliability of the detection of the FSR® activation.
It is also known from the document U.S. Pat. No. 5,440,237, a method and apparatus for normalizing electronic sensor data to correct for variations in individual sensor transfer characteristics that are not known in advance. A general characteristic transfer function of sensor type of interest is determined empirically. For that purpose, a baseline response is acquired from each sensor to get an indication of the transfer characteristics of each individual device. The baseline response is determined under some “preloaded” condition or “at rest” condition. Then a specific transfer function is determined for each individual sensor by applying the corresponding baseline response to the general characteristic transfer function. As for the previous document, it results from the above method that detection of the sensor activation does not take into account the environment of the sensor.
It is also known from the document US 2006/007172, a force sensing resistor with a calibration feature. For that purpose, it comprises the steps of measuring a calibration resistance while the FSR® is disconnected and determining a correction factor such that it is the ratio between the nominal value and the current measured value. Subsequent measurements of the FSR® resistance are then multiplied by the correction factor in order to scale them to appear as if they were measured from a nominal FSR® resistance.
In existing solutions, electronic measures, through a microprocessor Analogic Digital Converter (ADC), a voltage or frequency that will be the image of the FSR® resistance/pressure. As the FSR® resistance variation is assumed to follow a 1/F law, F being the force applied, thus the output voltage or frequency is a straight line as shown on FIGS. 1B and 1C.
The relation between the pressure applied on FSR® and the resistance variation is given on FIG. 1A. Therefore, theoretically whatever the resistance is, for a constant force ΔF, there is constant voltage ΔV as shown in FIG. 1B or a constant frequency Δf as shown in FIG. 1C.
Mainly used algorithms are generally based on high pass filter with long time constant (16 samples@20 ms sampling period). Further, the output value of this filter that depends on the velocity and force of the actuation is compared to thresholds for detecting any change on the sensor.
Another document, US 2009/066673, describes a self-calibration method of a pressure sensor. This method consists in periodically calibrating the sensor when not activated, determining an idle tension according to the current and previous values, compensating the measured value by compensation data and activating the sensor according to an updated threshold value. The compensation data may be in particular the relation between the measured voltage and the force applied on the sensor.
It is further known from the document U.S. Pat. No. 5,514,040 calibration methods of FSR® sensors. One of these methods is based on a delay value calibration, while another one is based on an adjustment value calibration. However, none of these methods takes into account the state of the push (released or pressed) and the mechanic structure of the sensor to make the calibration. Another document EP 0535907 describes a calibration method of a push button based on a measure done while the button is pressed by the user. Another document U.S. Pat. No. 6,456,952 describes a calibration method of a touch screen including key areas and drift areas. Following the detection of contacts in key areas and surrounding drift areas, an adjustment vector is applied to focus detection. Still another document, EP 602907 describes a calibration method based on two extreme measures, one measure without pressure and another measure with a maximum pressure. Finally, another document US 2007/107487 describes a generic calibration method.
Each of the above presented solutions present some of the following several drawbacks. In particular, the above presented systems will be influenced by electromagnetic interference (EMI)/compatibility (EMC). Indeed, protection stage against automotive EMI/EMC constraint will influence the voltage input value. Consequently, the input voltage is no more linear over the whole RFSR® variation range. Further, the mechanical environment, temperature and humidity have an influence on the mechanical pre-load system and FSR® intrinsic characteristics. Indeed, FSR® resistance variation is not really in 1/x, but in z/x (with 0.5<z<1.5) due to the mechanical structure of the sensor which is not a straight line over the whole resistance variation range. Moreover, it appears that high dynamic pressure detection is not reliable since with a high pass filter, the low velocity activation cannot be detected (actuation duration has to be less than the time constant, which may be not compliant with sensor requirements). Known algorithms do not take into account dynamic variation of preload detection. Indeed, due to mechanical warping on mechanical parts or on the FSR® sensor itself, the preload applied on the sensor can dynamically change. For instance, between two consecutive presses, the FSR® resistance level may change. Furthermore, current solutions do not take into account fast variations of the system such as dynamic variation of preload detection due to mechanical warping on the sensor that may change the for instance the sensor resistance level between two presses.